Applying differential InSAR to orbital dynamics: a new approach for estimating ERS trajectories

A new approach for tuning the trajectories of the European remote sensing (ERS) satellites is developed and assessed. Differential dual-pass interferometry is applied to calculate interferograms from the phase difference of synthetic aperture radar (SAR) images acquired by the ERS satellites over the site of the 1992 earthquake in Landers, California. These interferograms contain information about orbital trajectories and geophysical deformation. Beginning with good prior estimates of the orbital trajectories, a radial and an across-track orbital adjustment is estimated at each epoch. The data are the fringe counts along distance and azimuth. Errors in the across-track and radial components of the orbit estimates produce fringes in the interferograms. The spacing between roughly parallel fringes gives the gradients in distance and azimuth coordinates. The approach eliminates these fringes from interferometric pairs spanning relatively short time intervals containing few topographic residuals or atmospheric artefacts. An optimum interferometric path with six SAR acquisitions is selected to study post- and inter-seismic deformation fields. In order to regularize the problem, it is assumed that the radial and across-track adjustments both sum to zero. Applying the adjustment approach to the prior estimates of trajectory from the Delft Institute for Earth-Orientated Space Research (DEOS), root mean squares of 7.3 cm for the across-track correction components and 2.4 cm for the radial ones are found. Assuming 0.1 fringes for the a priori standard deviation of the measurement, the approach yields mean standard deviations of 2.4 cm for the across-track and 4.5 cm for the radial components. The approach allows an ‘interval by interval’ improvement of a set of orbital estimates from which post-fit interferograms of different time intervals spanning a total 3.8-year inter-seismic time interval can be created. The interferograms calculated with the post-fit orbital estimates compare favorably with those corrected with a conventional orbital tuning approach. Using the adjustment approach, it is possible to distinguish between orbital and deformation contributions to interferometric SAR (InSAR) phase gradients. Surface deformation changes over an inter-seismic time interval longer than one year can be measured. This approach is, however, limited to well-correlated interferograms where it is possible to measure the fringe gradient.